(The picture is an uncompressed .bmp so it is easy to download and
save as a test image. The original purpose of one-liner pictures was fast and easy test
image creation.)

Examples

Copy/paste these into the form then edit to produce variants.

Three ways to make an impulse

(x == 0 && y == 0) ? 1 : 0

x || y ? 0 : 1

!(x || y)

Notice that all generated pictures are re-ranged to [0,255].

Green diamond

abs(x)+abs(y) < 80 ? green : 0

Circular cosine

cos(sqrt(x*x+y*y)/10.0)

Because of re-ranging, this is just as good as:

128 + 127*cos(sqrt(x*x+y*y)/10.0)

A coloured field

abs(x-y)*red + (127-x)*green + (y+128)*blue

A [-green,green] ramp

y*green

Why does a green ramp have other colours? Because the range of values of
y is [-128,127], so when the picture is re-ranged, 128 gets added to all of
red, green and blue. If you want a pure green ramp, you have to do:

(y+128)*green

Uniformly distributed noise

rand() % 256

(The random number generator used here is believed to be random in
its lower-order bits.)